package com.example.leetcode;

/**
 * 给你单链表的头节点 head ，请你反转链表，并返回反转后的链表。
 *
 *
 * 示例 1：
 *
 *
 * 输入：head = [1,2,3,4,5]
 * 输出：[5,4,3,2,1]
 * 示例 2：
 *
 *
 * 输入：head = [1,2]
 * 输出：[2,1]
 * 示例 3：
 *
 * 输入：head = []
 * 输出：[]
 *
 *
 * 提示：
 *
 * 链表中节点的数目范围是 [0, 5000]
 * -5000 <= Node.val <= 5000
 *
 *
 * 进阶：链表可以选用迭代或递归方式完成反转。你能否用两种方法解决这道题？
 *
 * 206 反转链表
 */
public class Leetcode0206 {

    public ListNode reverseList(ListNode head) {
//        return reverseWithIterate(head);
        return reverseWithRecursive(head);
    }

    private ListNode reverseWithRecursive(ListNode head) {
        if (head.next == null) {
            return head;
        }
        ListNode rev = reverseWithRecursive(head.next);
        head.next.next = head;
        head.next = null;
        return rev;
    }

    private ListNode reverseWithIterate(ListNode head) {
        if (head == null) {
            return head;
        }

        ListNode pre = null;
        ListNode cur = head;
        ListNode next = cur.next;
        cur.next = pre;
        while (next != null) {
            pre = cur;
            cur = next;
            next = next.next;
            cur.next = pre;
        }
        return cur;
    }
}


class ListNode {
     int val;
     ListNode next;
     ListNode() {}
     ListNode(int val) { this.val = val; }
     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 }